In high frequency integrated circuit applications, such as used in wireless portable electronic devices, there is a demand for miniaturisation, greater functionality, higher performance, and lower cost, which can be achieved through the integration of passive components such as inductors and transformers in integrated circuits. The lack of passive component integration on integrated circuits contributes to a passives-to-actives component ratio as high as 100:1 in typical electronic systems. However, current passive components, when integrated with active elements on integrated circuits consume large areas of silicon, making their integration often not economical. Therefore, methods are developed to reduce the space used up by passive components on integrated circuits.
One approach to reduce the area of an inductor in an integrated circuit is through the integration of magnetic materials, for example materials that have a permeability greater than one. The aim of this approach is to increase the inductance per unit area through taking advantage of the increased electro-magnetic energy storage capability of high-permeability materials. The associated size reduction of the inductor enables enhanced performance (higher Q-factor) through reduced series resistance and reduced parasitic capacitance. In transformers, the same materials properties enable an increased coupling coefficient, hence increasing the energy transfer in a transformer while also enabling smaller area through confining the electromagnetic field lines within the magnetic material, hence allowing increased device packing density. The traditional method for achieving such passive device improvements is using amorphous or crystalline solid thin film magnetic materials, such as for example as disclosed in EP0716433. However, at high frequency applications, for example high MHz or low GHz frequencies, methods such as this have performance limitations due to eddy current losses and loss-generating ferromagnetic resonance (FMR).
An attempt to overcome these associated deficiencies has been to spatially separate and electrically isolate magnetic nanoparticles, i.e. particles of nanometer size, such that the material is ferromagnetic at device operating temperatures, and preferably being composed of a single magnetic domain. Since eddy current losses can be suppressed and the FMR decay extended to higher frequencies, the magnetic nanoparticles allow an increase in inductance density per unit area in the MHz-GHz range. It is important, however, that the composite magnetic nanoparticle material has properties such as a magnetic permeability (μ), i.e. greater than 1 (one), and corresponding losses (μ″) such that the quotient of (permeability)/(loss) is, for example, greater than 5 in the MHz—GHz frequency range of interest.
Several methods have been attempted to produce nanoparticles with some of these qualities, and generally fall in three categories: physical methods, template methods and chemical methods.
Physical methods such as sputtering and epitaxy (See for instance Y. M. Kim et al, IEEE Trans. on Magn. vol. 37, no. 4, 2001, and M. Dumm et al, Journal of Applied Physics vol. 87, no. 9, 2000, respectively) produce thin layers. However, both methods produce high permeability films only up to approximately 500 MHz and generally cannot be thicker than 2-3 micrometers in order to minimise eddy current losses. However, such performance is not sufficient for an effective magnetic field confinement needed to enhance passive devices.
Template methods such as the one described by Cao, H., Xu, Z., Sang, H., Sheng, D., Tie, C. Adv. Mater. 2001, vol. 13, p. 121 grow, usually electrochemically, nanorods or nanowires within the channels of inorganic or track-etched organic matrices. The main drawback of this method comes from the destruction of the organic or inorganic matrix after the particle formation that yields by-products, which generally interfere with the nanoparticles. This results in a decrease of the saturation magnetization compared to that of bulk metal and may also prevent the formation of high density material.
Chemical methods like chemical reduction or decomposition of a carbonyle precursor such as described by Sun, S. Murray, C. B. J. Appl. Phys. 1999, vol. 85, p. 4325 or by Alivisatos, P. Puntes, V. F. Krishnan, K. M. Appl. Phys. Lett. 2001, vol. 78, p. 2187 involve synthesis of nanoparticles in solutions. The chemical methods, and more specifically the method of Sun et al. results in the production of self-assembled monodisperse spherical cobalt particles from a carbonyl cobalt precursor for use in high-density recording. However, due to their small size, the particles remain superparamagnetic at room temperature, and hence will not generate the required magnetic field confinement as achieved by high permeability materials. Additional work has been disclosed by the same authors in Sun, S. Murray, C. B. Weller, D. Folks, L. Moser, A. Science 2000, vol. 287, p. 1989 generating iron-platinum (Fe/Pt) particles to enhance the magnetic anisotropy of the material. However, similarly to above, the small size of the particles in the material makes them superparamagnetic, hence not useful for magnetic field confinement. In Puntes, V. F. Krishnan, K. M. Alivisatos, A. P. Science 2001, vol. 291, p. 2115 cobalt nanorods are produced from a carbonyl cobalt precursor by using a mixture of oleic acid and trioctylphosphine oxide (TOPO). However this requires a high temperature process of, for example, approximately 300° C. Moreover, the nanorods obtained in this way are not thermodynamically stable and typically spontaneously rearrange into spherical nanoparticles within the first few seconds of the reaction. Iron and Nickel nanorods have also been recently reported through decomposition of, respectively Fe(CO)5 (Park, S.-J.; Kim, S.; Lee, S.; Khim, Z. G.; Char, K.; Hyeon, T. J. Am. Chem. Soc. 2000, vol. 122, p. 8581) and Ni(COD)2 (N. Cordente, C. Amiens, F. Senocq, M. Respaud, and B. Chaudret, Nano Letters 2001, 1(10), p. 565) in the presence of ligands such as TOPO and hexadecylamine (HDA). Both materials are not homogeneous (particles do not display the same shape). Furthermore, these particles are superparamagnetic at room temperature and hence unsuitable for magnetic field confinement.
Thus, there is a need for a magnetic material consisting of magnetic nanoparticles that are ferromagnetic at room temperature and/or operating temperatures up to for example approximately 105° C., and of homogeneous size, shape, and magnetic orientation. Furthermore, there is a need for a method of making such thermodynamically stable magnetic nanoparticles of adjustable aspect ratio encapsulated within a non-magnetic matrix, so that such final magnetic nanoparticle materials could be used in high frequency integrated circuit applications, such as used in wireless portable electronic devices, to enhance magnetic field confinement in a variety of passive and active devices.